Good afternoon everyone its Friday and I hope everyone is having a great day. Its been a little gap in time between series parts due to some technical issues. I am going to blog about Baroclinic Systems in detail today. The main system in the baroclinic features are the Mid-Latitude Cyclone, which will be discussed in much detail. I am briefly going to discuss the Baroclinic High Pressure System as well. Let’s Begin. The next blog will cover 500mb changes During cyclogenesis this involves VORTICITY.
1. INTRODUCTION –
Baroclinic Systems are driven by Baroclinic Instability, that is a sharp contrast in temperature, or temperature differential. Baroclinic Instability relies on thermal advection to mix gradients(dewpoint, temperature, etc). If lapse rates are steep where there is strong surface heating and temperatures aloft at 700-500 mb are considerably colder than surface temperatures, instability is clearly evident and in place. This is much different compared to a barotropic environment where homogenous temperatures are evident throughout the atmospheric profile. Okay with that foundation being set, let’s get into the systems of the Baroclinic Weather Systems.
2. BAROCLINIC HIGH PRESSURE –
Baroclinic High Pressure Systems can not support fronts(cold,warm,etc). the reason is diverging air weakens thermal gradients, basically it spreads the thermal gradients out. Baroclinic Highs develop in cold air masses, behind the Mid-Latitude Cyclone. The air that comprises the baroclinic high pressure system eventually warms due to subsidence, which is sinking air, and heat modification. the air becomes barotropic because the entire atmospheric temperature profile is about the same, and it is absorbed by the Subtropical High Pressure Ridge(Bermuda). Baroclinic High Pressure Systems strengthen under confluent flow by the Jetstream, which is air merging together above High Pressure. This occurs especially north of the Jetstream. Baroclinic Highs weaken south of the Jetstream because of the two points I mentioned previously, subsidence and heat(modification of the air mass).
Here is a map by Plymouth State Weather Center showing a very large Baroclinic High Pressure advecting and providing cold air for a major blizzard along the Northeast Coast in late January of 2005. Notice how the High is situated NORTH of the Polar Front Jetstream. This is the Essence of a Baroclinic High Pressure System.
3. BAROCLINIC LOW PRESSURE –
I am going to talk about cyclone development in this blog entry. the next blog will delve into the stages of the Mid-Latitude Cyclone because it will be too long. this is going to get technical so if there is any questions please ask me through the comment section, email, or my iPhone. Ok let’s begin.
A. Cyclone Development – A quick not here – except during the cases of explosive cyclogenesis at the surface, the evolution of the 500mb trough into a closed cyclone takes place more slowly at the surface. At the mid-levels low pressure at more wave like, while at the surface low pressure is cellular. Low pressure at the surface close themselves quicker and easier than at mid-levels(500mb). So to recap Low pressure at the center located at the surface close themselves easier and quicker, but a trough still develops at the mid levels and a deep trough is developed at the mid and upper levels of the atmosphere.
B. 500mb Cyclone Development – Unlike the Barotropic Model, we must account for low-level thermal advection( thermal meaning temperature and/or heat), causing Lift and Vorticity Generation. Much of the time, 500mb vorticity changes are due to advection or the movement of thermal air masses. Development of Troughs at 500mb is MOST intense on the Cold Side of Cold Fronts. Thermal advections are largest or steepest here. Furthermore, development of the 500mb troughs occurs East of the surface anti-cyclone(High Pressure Ridge) in the region where there is surface pressure rises and therefore low level divergence(sinking air), decreasing height lines, therefore a development of a 500 mb trough.
C. Baroclinic Development – this next section will discuss heights and temperature profiles.
Negative 500mb height tendencies and positive surface pressure tendencies occur West and Southwest of the surface low pressure in association with Cold Air Advection behind the cold front. Warm Air Advection occurs South of the Warm front and East of the Cold front. So in the wake of the warm from at the surface low pressure, temperatures rise and humidity and dewpoint rises as well. When the cold front passes temperatures drop, humidity and dewpoint drops as well. Severe weather can occur out ahead of the cold front or along it. Strong gradients occur on the cold side of cold fronts.
Mid Level Trough – Associated with dark solid lines on the 500mb geopotential height map. A very important point I remember this from my synoptic class is in the coldest advections, where the cold air is the STRONGEST, this is where the trough is DEEPEST. Lines on the geopotential height map lag South and Southwest of the center of the Surface Low pressure System. There is a Vertical Tilt so the surface low is displaced to the North and Northeast of the deepest part of the trough. In essence Mid-level heights decrease, and surface pressure lines increase south and west of the surface cyclone. This region is located just North and Northeast of the deepest trough. Both of these are associated with COLD AIR ADVECTION. This process works to enhance cyclogenesis. Likewise, Positive 500mb height tendencies and negative surface tendencies occur NORTHEAST of the surface cyclone associated with WARM AIR ADVECTION. In the region of warm air advection, pressures are decreasing due to the surface low heading in that direction. The Pressure rises where the Surface Low is at its present location. Just a quick note to people interested in weather or Meteorologists, if you look at the 700mb charts to see where Upper Vertical Velocities are present, this is a good indication to where a Surface Low is headed. Follow the Rising Air!! Technically speaking, in the geographic location where warm air advection is taking place, negative surface pressure tendencies are occurring EAST and NORTHEAST of the Surface Low, this is where the Low Pressure is going.
Phase Lag – This condition is necessary for 500 mb development. For 500mb cyclone development to occur a PHASE LAG occurs between thickness at the surface and 500mb WAVES with colder air lagging behind the surface trough.
Coupled Development – This is essential for trough and Ridge development. this process usually accompanies rapid cyclogenesis at the surface in the regions of STRONG Warm Air Advection and Cold Air Advection in the lower troposphere. The region of Strongest Cold Air Advection at the mid-levels is where troughs occur!! The region of Strongest Warm Air Advection at the surface results in a Ridge!! Maximum 500mb Height falls occur Southwest of the Surface Low, West of the Cold Front in the region of the Strongest Cold Air Advection. Strongest height rises occur poleward(North) of Low Pressure in the region of strongest Warm Air Advection. I can get into absolute vorticity, positive and negative vorticity but I am going to hold off. I am going to focus on the thermal advection of baroclinic development here. My time is limited however the last section will involve thermodynamics.
D. Temperature Gradient –
This is the most important part of Baroclinic Low Pressure Development. The key to rapis development in the cyclone is intense temperature gradient. This temperature gradient provides the initial forcing. This leads to strong ascent and low-level convergence and upper level divergence. This also contributes to 500mb height falls in the region of Cold Air Advection. This temperature gradient contrast is at the heart of the process known as Baroclinic Instability, which is manifested in the VERTICALLY coupled cyclone development.
source theweatherprediction.com
As you can see this is a mid-latitude cyclone or a baroclinic low pressure system. The front north of the Low is the warm front, that has semi circles associated with it, is moving North to Northeast and the front south and west of the low is the cold front, which is moving South and Southeastward. Each front is advecting their airmass with them.
The baroclinic leaf is a giant leaf cloud structure depicted on the map above, the leaf corresponds to major confluence zones, winds that are merging along a single axis. The Baroclinic leaf may extend over large distances crossing over geopotential height lines. The leaf can be situated on the anti-cyclonic side of a Jetstream, near a Ridge of High Pressure. Sometimes they are between a Trough and a Ridge. It is a Precursor to intense cyclogenesis. A Baroclinic Leaf is a typical case of a deep trough and a strong polar Jetstream with NO well defined surface cyclone. On a weather analysis map a baroclinic Leaf has very sharply defined edges. Cold and dry from the West and North merging with warm and moist air from the South and East.
Well that is it for today I hope everyone liked this blog, I try to make it educational and I hoped everyone has learned something today and in the series of blogs on barotropic and baroclinic weather systems. I would like to thank my good friend Jeff B the weather professor and everyone else that reads my blog entries. if there are any questions I will try to answer them as fast as I can. Thank You all!